1. Field of the Invention
This invention relates to the three dimensional imaging of objects using a single pulse of energy.
2. Description of the Prior Art
A previous U.S. Pat. No. 4,688,430, describes a similar machine. However in that application the reconstruction process was carried out in two geometrical steps. In the first step the image was resolved with respect to each of two angular directions originating at the transmitting transducer. In the second step the ranges along each two dimensional angular direction are resolved. This required a complicated mathematical algorithm to be implemented in the reconstruction hardware, and in addition, mathematical approximations to the wavefield geometry may be required.
The previous patent essentially uses spherical coordinates originating at the center of the transmitting transducer. This yields a lateral resolution that is very small near the transducer but grows larger as the distance from the transducer increases. However, a large aperture formed by a sparse array is capable of producing relatively uniform resolution for ranges equal to several diameters of the array. The previous patent shows an intermediate memory called the data memory which is used to store the three dimensional reconstructed field of reflecting objects. This is an intermediate memory since it must be further processed into a tomographic image or a two dimensional view through a three dimensional field of objects. The present invention can reconstruct tomograms or two dimensional views through three dimensional fields directly from the stored time history of the receiver elements. This increase the processing speed and reduces the amount of electronics by eliminating the requirements for an intermediate memory.
The final display will most likely be in rectangular coordinates whereas the previous approach reconstructs in spherical coordinates. This results in complex electronics being required in the implementation of the machine.
The transmitted wavefield from a small transducer emitting a pulse will approximate an expanding sphere at locations several diameters away from the transducer. This must be taken into account by the reconstruction processor or image degradation will occur. The previous approach makes no explicit provision for this and it would be hard to implement in such a two step reconstruction technique. The implementation would require look up tables or computation circuits for each reconstruction point and each receiver element. This would be hard to implement in real time and would use a large amount of electronics.
The present invention uses a round trip time of flight algorithm which automatically takes into account the curved nature of the wavefront propagating away from the transmitter. In addition no Frauhofer or Fresnel approximations are made since the algorithm is essentially a Huygens wavelet based approach. It requires only the computation of the distance from the transmitter to the reconstruction point and computation of the distance from the reconstruction point to each receiver element.
The parent application describes an improved imaging technique whereby the foregoing disadvantages are overcome. However, there are further improvement that can be made.
The time history memories can be eliminated by a reconstruction techniques that immediately sums or combines echo samples as they are sampled into the appropriate reconstruction locations (or voxels) in the 3D memory containing the reconstructed three dimensional image.
The 3D memory can be eliminated by combining the echo samples as they are sampled into the appropriate pixels in the tomographic image and the shadowgraph image. (The shadowgraph image is the 3memory data integrated along a specified viewing perpective vector to provide a two dimensional view through a three dimensional volume)
Multiple redundant transmitted pulses may be used to reconstruct a single image whereby the image signal to noise ratio is improved. The term "redundant" is used since only one transmitted pulse is necessary for the reconstruction of a three dimensional image.
Multiple transmitters may be used with the redundant transmitted pulses to reduce the sidelobe levels. These transmitters are to be spatially offset from one another. The receiver elements may be used as the multiple transmitters or separate transmitting elements may be used. The echoes from the multiple transmitters may be summed in time history memories or separate images may be reconstructed from each different transmitters echoes and the resulting images combined or summed. The later may be done in the absence of time history memories.
Recording devices may be added to record the echo time histories from a number of sequential transmitted pulses, at a later time the recording then can be played back though the machine allowing image reconstruction to take place. The resulting "real time" 3D image can be viewed from various viewing perspectives and tomograms may be extracted from various positions and orientations.
If oscillations occur in the transmitted pulse, the image will be degraded. Several techniques may be used to compensate for or accomodate this.
The reconstruction technique described in U.S. Pat. No. 4,706,499 is essentially the backprojection of the echo samples over ellipsoids of revolution as will be more fully described in this application. The backprojections may be weighted as a function of the reconstruction point position to compensate for transmitter or receiver radiation patterns and other phenomena.
The sparse receiver array, by the addition of elements, may be made into a more nearly continous array which when arranged in a circle would be a phased annulus or adjustable axicon. This sort of receiver array normally has very high sidelobes but when used with a noninterfering transmitted pulse has accepatable sidelobe levels. The addition of redundant pulsing and multiple transmitters further reduces the sidelobe level.
In forming shadowgraphs by integration (two dimensional views through three dimensional volumes), the sidelobes are integrated and the relative sidelobe level is degraded. After a three dimensional image is created of a volume containing many point reflectors, the sidelobes create a more or less continuous backround level. If this background level is subtracted out (or truncated) before the shadowgraphs are created, the relative sidelobe level will not be degraded as much.
Another method of reducing sidelobe levels is to use a nonlinear form of combination in the reconstruction process (as contrasted with only using addition). A nonlinear form of combination may be used when forming the shadowgraphs in place of integration.